International observers issued a stark warning over escalating violence in eastern Ukraine as Dutch investigators hoping to recover debris from a doomed airliner left the MH17 crash site with little progress.

The Organisation for Security and Cooperation in Europe said there was a "rising" risk that unrest in the war-torn region would get worse amid fears of a return to an all-out conflict.

The Ukrainian envoy to the United Nations said meanwhile that Moscow was planning an all-out offensive.

A fragile ceasefire has been in place in the east of the former Soviet republic since September but has been regularly shaken by localised outbreaks of fighting.